Curable silicone compositions that are cured by hydrosilylation reaction to form cured products having a high refractive index and high light transmittance have been used as protective agents, coating agents, or sealing agents for semiconductor elements in optical semiconductor devices such as photocouplers, light emitting diodes (LEDs), solid-state image sensing devices, or the like. As such a curable silicone composition, a curable silicone composition comprising: a straight-chain organopolysiloxane having, in a molecule, at least two alkenyl groups and at least one aryl group; a branched-chain organopolysiloxane having, in a molecule, at least one alkenyl group and at least one aryl group, and having siloxane units represented by general formula: RSiO3/2 (in the formula, R is a substituted or unsubstituted monovalent hydrocarbon group); an organopolysiloxane having, in a molecule, at least two silicon atom-bonded hydrogen atoms; and a hydrosilylation catalyst has been known (see Patent Document 1).
As organopolysiloxanes having a silicon atom-bonded hydrogen atom, straight-chain, branched-chain, or resin-like organopolysiloxanes have been used; however, to enhance mechanical characteristics or the like of the resulting cured product obtained by curing a curable silicone composition, Patent Documents 2 and 3 propose use of an organopolysiloxane that is composed of an organopolysiloxane resin block and a diorganopolysiloxane block, and that has a resin structure having a silicon atom-bonded hydrogen atom.
Patent Documents 2 and 3 describe production of an organopolysiloxane by subjecting an organotrichlorosilane, a diorganopolysiloxane in which silicon atoms at both molecular terminals are capped with halogen atoms, chlorosilane having a silicon atom-bonded hydrogen atom, and chlorosilane having an alkenyl group to a co-hydrolytic condensation reaction. However, such methods cannot prepare an organopolysiloxane having a siloxane residue or silphenylene residue having a silicon atom-bonded hydrogen atom in an organopolysiloxane resin block.
Meanwhile, Patent Document 4 describes a branched-chain organopolysiloxane having a siloxane residue having a silicon atom-bonded hydrogen atom as the organopolysiloxane of Formula 16. However, in Patent Document 4, an organopolysiloxane having a siloxane residue or silphenylene residue having a silicon atom-bonded hydrogen atom in an organopolysiloxane resin block is not suggested.
Furthermore, Patent Document 5 describes preparation of a solvent-soluble organopolysiloxane by subjecting an organopolysiloxane resin having an alkenyl group and a diorganopolysiloxane having silicon atom-bonded hydrogen atoms at both molecular terminals to a hydrosilylation reaction at an amount such that the amount of the silicon atom-bonded hydrogen atoms in the diorganopolysiloxane is less than 1 mol per 1 mol of alkenyl groups in the organopolysiloxane resin. However, in Patent Document 5, an organopolysiloxane having a siloxane residue or silphenylene residue having a silicon atom-bonded hydrogen atom in an organopolysiloxane resin block is not suggested.